The present invention relates generally to gas turbine engines, and more particularly, to turbofan aircraft engines.
One of the primary design criteria for aircraft turbofan engines is to propel an aircraft in flight with maximum efficiency, thereby reducing fuel consumption. Thus, turbofan engines are continually being developed and improved to maximize thrust capability with the greatest aerodynamic efficiency possible.
One of the components of the turbofan engine is an array of fan blades, which are positioned adjacent the forward portion of the turbofan adjacent the turbofan's inlet. The fan blades produce thrust, and thus, are typically designed to maximize the aerodynamic loading and the amount of propulsion thrust generated thereby during operation. However, fan loading is limited by stall, flutter, or other instability parameters of the air being pressurized.
Fan stall margin is a fundamental design requirement for the turbofan and is affected by aerodynamic fan loading. A major factor affecting the aerodynamic loading of the fan is the geometry of the inlet upstream of the fan. Aircraft wings are known to induce an upwash velocity in airflow in the inlet. Unfortunately, conventional inlets do not account sufficiently for the vertical component (vector) of the airflow. The circumferential vector of the airflow causes the fan to operate with different circumferential sectors having different flow-pressure ratio characteristics. Specifically, the circumferential vector of the inlet airflow manifests itself as a swirl velocity along the face of the fan blades. The swirl velocity is in a direction counter to the direction of rotation of the fan along at least a portion of the fan face. The swirl velocity has a destabilizing effect on the flow condition over sector(s) of the fan, and thus, degrades the stall margin of the fan.
Similarly, the different flow-pressure ratio characteristics on the fan produce a circumferential variation in total pressure at the inlet to the core stream of the turbofan. The total pressure variation (distortion) has a destabilizing effect on the operation of the low pressure compressor and high pressure compressor sections of the turbofan.